Pulmonary hypertension (PH) is an increase in blood pressure in the pulmonary artery, pulmonary vein, or pulmonary capillaries, together known as the lung vasculature.
It is a disease phenomenon of multifactorial etiology with high mortality. The disease causes increased work for the right side of the heart and eventually hypertrophy and dysfunction, often in both the right and left side of the heart. Despite improvements in therapy, the prognosis of pulmonary hypertension is poor, with median survival being around 5 years.
Typically, patients suffering from PH are treated with pharmacological agents which are extremely expensive (as much as $100,000 per year for an adult) and still not entirely-efficacious. Moreover, treatment outcome amongst patients suffering from PH is highly variable, mainly due to the variance in underlying factors.
A distinction has been made between primary and secondary PH.
Primary PH (WHO group I PH—Pulmonary arterial hypertension (PAH)) involves the vasoconstriction or tightening of blood vessels connected to and within the lungs. This makes it harder for the heart to pump blood through the lungs. Over time, the increased workload of the heart causes hypertrophy of the right ventricle, making the heart less able to pump blood through the lungs, ultimately causing right heart failure.
Secondary PH can again be divided into four forms of PH, each having a distinct underlying etiology.
WHO Group II PH refers to a secondary PH in which the PH is owing to left heart disease. The pathogenesis in WHO Group II PH is entirely different from that of WHO Group I in that constriction or damage to the pulmonary blood vessels is not the issue. Instead, the left heartfails to pump blood efficiently, leading to pooling of blood in the lungs and back pressure within the pulmonary system. This causes pulmonary edema and pleural effusions.
WHO Group III PH refers to a secondary PH in which the PH is owing to hypoxia. The low levels of oxygen are thought to cause vasoconstriction or tightening of pulmonary arteries, hypoxic pulmonary vasoconstriction. When prolonged, the hypoxia-mediated vasoconstriction occurs across a large portion of the pulmonary vascular bed and eventually brings about pulmonary hypertension. Non-limiting examples of WHO group III PH include chronic obstructive pulmonary disease (COPD) associated PH and interstitial lung disease (ILD) associated PH.
WHO Group IV refers to a secondary PH in which the PH is owing to chronic thromboembolic pulmonary hypertension. In this case, the blood vessels are blocked or narrowed with recurrent blood clots, which often cause the blood vessels to constrict. This combination of blocked or narrowed vessels and vasoconstriction once again increases the resistance to blood flow and so the pressure within the system rises.
WHO Group IV refers to a secondary PH in which the underlying cause of the PH is unknown.